JP5672525B2 - Transfer device and image forming apparatus - Google Patents

Description

  The present invention relates to a transfer device that transfers a toner image on the surface of an image carrier to a recording material sandwiched in the nip in a transfer nip formed by contact between the image carrier and a nip forming member, and an image using the transfer device. The present invention relates to a forming apparatus.

  As this type of image forming apparatus, the one described in Patent Document 1 is known. This image forming apparatus forms a toner image on the surface of a drum-shaped photoreceptor by a known electrophotographic process. A primary transfer nip is formed on the photoreceptor by contacting an endless intermediate transfer belt as an image carrier. Then, in the primary transfer nip, the toner image on the photoconductor is primarily transferred to the intermediate transfer belt. A secondary transfer nip is formed on the intermediate transfer belt by contacting a secondary transfer roller as a nip forming member. Further, a secondary transfer counter roller is disposed in the loop of the intermediate transfer belt, and the intermediate transfer belt is sandwiched between the secondary transfer counter roller and the above-described secondary transfer roller. While the secondary transfer counter roller inside the loop is connected to the ground, a secondary transfer bias is applied to the secondary transfer roller outside the loop. Thereby, a secondary transfer electric field for electrostatically moving the toner image from the secondary transfer counter roller side to the secondary transfer roller side is formed between the secondary transfer counter roller and the secondary transfer roller. The toner image on the intermediate transfer belt is secondarily transferred to the recording paper fed into the secondary transfer nip at a timing synchronized with the toner image on the intermediate transfer belt by the action of the secondary transfer electric field or nip pressure. To do.

  In such a configuration, when recording paper having a large surface irregularity such as Japanese paper is used, it becomes easy to generate a light and shade pattern in the image according to the surface irregularity. This light and shade pattern is generated when a sufficient amount of toner is not transferred to the concave portion on the paper surface and the image density of the concave portion becomes lighter than that of the convex portion. Therefore, in the image forming apparatus described in Patent Document 1, as the secondary transfer bias, not only a DC voltage but also a superimposed bias in which a DC voltage is superimposed on an AC voltage is applied. In Patent Document 1, by applying such a secondary transfer bias, it is possible to suppress the occurrence of a light and dark pattern as compared with the case where a secondary transfer bias consisting only of a DC voltage is applied.

  However, the present inventors have found through experimentation that in such a configuration, it is easy to generate a plurality of white spots at an image portion formed on a concave portion on the paper surface. Therefore, the present inventors have conducted extensive research on the cause of white spots, and have found the following. That is, FIG. 1 is an enlarged configuration diagram showing an example of the secondary transfer nip. In the drawing, the intermediate transfer belt 531 is pressed toward the nip forming roller 536 by a secondary transfer back roller 533 in contact with the back surface thereof. By this pressing, a secondary transfer nip where the front surface of the intermediate transfer belt 531 and the nip forming roller 536 are in contact is formed. The toner image on the intermediate transfer belt 531 is secondarily transferred onto the recording paper P fed into the secondary transfer nip. A secondary transfer bias for secondary transfer of the toner image is applied to one of the two rollers shown in the figure, and the other roller is grounded. It is possible to transfer the toner image to the recording paper P regardless of which roller the transfer bias is applied to, but it is a case where the secondary transfer bias is applied to the secondary transfer back surface roller 533 and as the toner. A case of using a negative polarity will be described as an example. In this case, in order to move the toner in the secondary transfer nip from the secondary transfer back surface roller 533 side to the nip forming roller 536 side, the time average value of the potential is the polarity of the toner as the secondary transfer bias composed of the superimposed bias. Apply the same negative polarity potential.

  FIG. 2 is a waveform diagram showing an example of the waveform of the secondary transfer bias composed of the superimposed bias applied to the secondary transfer back surface roller 533. In the figure, the offset voltage Voff [V] represents the time average value of the secondary transfer bias. As shown in the figure, the secondary transfer bias composed of the superimposed bias has a sine wave shape, and has a positive peak value and a negative peak value. Of these two peak values, the sign Vt is the peak value of the one that moves the toner from the belt side to the nip forming roller 536 side in the secondary transfer nip (minus side in this example). Yes (hereinafter referred to as feed peak value Vt). Also, the symbol Vr is a peak value (hereinafter referred to as a return peak value Vr) of the toner returning from the nip forming roller 536 side to the belt side (in this example, plus side). Even if an AC bias consisting only of an AC component is applied instead of the superimposed bias as shown, it is possible to reciprocate the toner between the belt and the recording paper in the secondary transfer nip. However, with AC bias, the toner cannot be transferred onto the recording paper simply by reciprocating. By applying a superimposed bias including a direct current component and setting the offset voltage Voff [V], which is a time average value thereof, to the same negative polarity as that of the toner, the toner is moved back and forth while relatively moving from the belt side to the recording paper side. It is possible to transfer to the recording paper.

  The inventors of the present invention observed the reciprocal movement with an experimental apparatus and found the following. That is, when the application of the secondary transfer bias is started, first, only a very small amount of toner particles existing on the surface of the toner layer on the intermediate transfer belt 531 are detached from the toner layer, and the concave portion on the surface of the recording paper is formed. Head in. However, most toner particles in the toner layer remain in the toner layer. The very few toner particles separated from the toner layer enter the recesses on the surface of the recording paper and then return to the toner layer from the recesses when the direction of the electric field is reversed. At this time, the reversing toner particles collide with the toner particles remaining in the toner layer, and weaken the adhesion of the toner particles to the toner layer (or recording paper). Then, when the electric field is next reversed in the direction toward the recording paper P, more toner particles than in the first time are detached from the toner layer and directed toward the concave portion on the surface of the recording paper. By repeating such a series of behaviors, the number of toner particles that are separated from the toner layer and enter the recesses on the surface of the recording paper is gradually increased, and a sufficient amount of toner particles are placed in the recesses. It was found that it was transferred.

  In the configuration in which the toner particles are reciprocated in this way, if the return peak value Vr shown in FIG. 2 is not set to a large value to some extent, the toner particles that have entered the recesses on the surface of the recording paper are sufficiently contained in the toner layer on the belt. The image density cannot be pulled back to the image, and the image density is insufficient on the concave portion. If the offset voltage Voff is not set to a certain large value, a sufficient amount of toner cannot be transferred to the convex portion on the surface of the recording paper, resulting in insufficient image density on the convex portion. In order to obtain a sufficient image density at both the convex and concave portions on the recording paper surface, the peak-to-peak voltage Vpp is set to a relatively large value in order to set the offset voltage Voff and the return peak value Vr to a certain large value. Must be set to Then, the feed peak value Vt is inevitably set to a relatively large value. The feed peak value Vt corresponds to the maximum potential difference between the grounded nip forming roller 536 and the secondary transfer back surface roller 533 to which the secondary transfer bias is applied. It becomes easy to generate. In particular, discharge is generated in a minute gap formed between the belt and the concave portion on the surface of the recording paper, and it becomes easy to cause a white spot in an image portion on the concave portion. In order to obtain sufficient image density at the convex portions and concave portions on the surface of the recording paper, the peak-to-peak voltage Vpp is set to a relatively large value so that white spots are formed at the image portions on the concave portions of the recording paper surface. It turned out that it was easy to generate.

  The present invention has been made in view of the above background, and an object of the present invention is to suppress the generation of white spots while obtaining sufficient image density at the concave and convex portions on the surface of the recording material. A transfer device and an image forming apparatus that can be used.

In order to achieve the above object, the invention of claim 1 is characterized in that a nip forming member that forms a transfer nip by contacting the front surface of the image carrier carrying the toner image is sandwiched in the transfer nip. In a transfer apparatus having transfer bias output means for outputting a transfer bias including a direct current component and an alternating current component for transferring a toner image on the image carrier to a recording material, the transfer bias is rectangular. Ri repeated Do from the rising pulse and the rectangular falling pulse shape, switching a plurality of times a bias polarity while passing the recording material to the transfer nip, of the previous SL rising pulse and the falling pulse , sustained towards the is one of the pulse generating an electric field in the direction of moving the toner is charged to the normal polarity from the image bearing member to the nip forming member side in the nip Between, longer than the duration of the other pulses of the toner image is charged to the polarity of the normal from the nip forming member side is moved to the image carrier-side, and the duration of the one pulse, the one pulse The combination of the size of the second pulse, the duration of the other pulse, and the magnitude of the other pulse is such that the toner charged to a normal polarity in the transfer nip is relatively transferred from the image carrier side to the nip forming member side. The transfer bias output means is configured to output a combination that is moved to the position.
According to a second aspect of the present invention, in the transfer apparatus according to the first aspect, the ratio of the duration of the other pulse to the sum of the duration of the rising pulse and the duration of the falling pulse is used as the alternating current component. The transfer bias output means is configured to output the output higher than 10 [%] and lower than 50 [%].
Further, in the transfer device according to claim 2, the transfer bias output in the transfer apparatus according to claim 2 so that the alternating current component is output with the duration of the other pulse being 0.2 [ms] or more. It is characterized by comprising means.
According to a fourth aspect of the present invention, in the transfer apparatus according to any one of the first to third aspects, a nip having a frequency f [Hz] and a length of the transfer nip in the moving direction of the image carrier surface as the AC component. The transfer bias output so that the width d [mm] and the surface moving speed v [mm / s] of the image carrier have a relationship of “f> (4 / d) × v” are output. It is characterized by comprising means.
According to a fifth aspect of the present invention, in the transfer device according to any one of the first to fourth aspects, the transfer bias output means is configured to perform a process of outputting the DC component by constant current control. It is what.
According to a sixth aspect of the present invention, in the transfer device according to the fifth aspect, the transfer bias output means is configured so as to carry out a constant current control process for the output value of the peak-to-peak current of the AC component. It is a feature.
According to a seventh aspect of the present invention, there is provided an image forming apparatus comprising: an image carrier that carries a toner image; and a transfer device that transfers a toner image on a surface of the image carrier to a recording material. The transfer device according to any one of claims 1 to 6 is used.
The invention according to claim 8 is the image forming apparatus according to claim 7, wherein the recording material has irregularities on the surface.

In these inventions, the alternating current component of the transfer bias is obtained by repeating a rectangular rising pulse and a rectangular falling pulse as compared with the conventional case of using a sinusoidal one. The toner is reciprocated at a lower peak-to-peak voltage. Specifically, in the conventional sinusoidal AC component, as shown in FIG. 2, there is a certain time lag from when the AC component voltage starts to rise to the plus side and reaches the plus side peak. There is also a certain time lag from when the voltage starts to fall to the minus side until it reaches the peak on the minus side. As a result, the time during which the voltage reaches the positive peak value or the negative peak value within one cycle of the voltage waveform is negligible. On the other hand, as in the present invention, when an alternating current component consisting of repetition of a rectangular rising pulse and falling pulse is used, the positive peak is reached at the moment when the voltage starts to rise to the plus side, and the voltage rises to the minus side. Since the peak on the negative side is reached at the moment when it starts to fall, the time to reach the peak is significantly longer than that of the sine wave. As a result, the toner can be reciprocated at a lower peak-to-peak voltage.
Further, in the present invention, of the rising pulse and the falling pulse, the pulse (hereinafter referred to as the feed pulse) that moves the toner charged with the normal polarity from the image carrier side to the nip forming member side is continued. By making the time longer than the duration of the other pulse (hereinafter referred to as the return pulse), the peak value of the feed pulse is made lower than when the durations of the pulses are the same as each other. It becomes possible. Specifically, regarding the feed pulse, even when a large amount of toner enters the transfer nip, such as when outputting a solid image, the large amount of toner is good from the image carrier to the recording material on the nip forming member side. It is necessary to set the conditions that can be transferred to. If the duration of the feed pulse is made relatively short, the peak value of the feed pulse needs to be made relatively large since a large amount of toner must be transferred from the image carrier to the recording material in a short time. . On the other hand, when the duration of the feed pulse is made relatively long, a sufficient amount of time is taken to transfer a large amount of toner from the image carrier to the recording material, so that the peak value of the feed pulse is made relatively small. It becomes possible. For this reason, by making the duration of the feed pulse longer than the duration of the return pulse, the peak value of the feed pulse can be made lower than when both are made the same length. For the return pulse, it is necessary to increase the peak value as compared with the case where the durations of both pulses are the same, but the amount is smaller than the amount for reducing the peak value of the feed pulse. This is because it is not necessary to return the entire amount of toner transferred to the recording material by the feed pulse to the toner layer on the image carrier by the return pulse, and it is sufficient to make a certain amount collide with the toner layer. For this reason, by making the duration of the feed pulse longer than the duration of the return pulse, the peak-to-peak voltage that is the sum of the feed peak value and the return peak value can be made smaller.
As described above, according to the present invention, the peak-to-peak voltage can be further reduced as compared with the conventional case. Therefore, while obtaining a sufficient image density at each of the concave and convex portions on the surface of the recording material, The generation of dots can be suppressed.

FIG. 3 is an enlarged configuration diagram illustrating an example of a secondary transfer nip. FIG. 6 is a waveform diagram showing an example of a transfer bias waveform composed of a superimposed bias. 1 is a schematic configuration diagram illustrating a printer according to an embodiment. FIG. 2 is an enlarged configuration diagram illustrating an enlarged image forming unit for K in the printer. The schematic block diagram which shows the observation experiment apparatus used for experiment. FIG. 4 is an enlarged schematic diagram illustrating the behavior of toner at an initial transfer stage in a secondary transfer nip. FIG. 5 is an enlarged schematic diagram illustrating the behavior of toner in the middle stage of transfer in the secondary transfer nip. FIG. 4 is an enlarged schematic diagram illustrating the behavior of toner at a late transfer stage in the secondary transfer nip. FIG. 2 is a block diagram showing a part of an electric circuit of the printer. FIG. 4 is a waveform diagram showing a voltage waveform of a secondary transfer bias output from a secondary transfer power source of the printer. The graph which shows the relationship between the offset voltage Voff and the image density on a paper surface convex part on the conditions of return time ratio = 20 [%]. The graph which shows the relationship between the offset voltage Voff and the image density on a paper surface convex part on the conditions of return time ratio = 50 [%]. The graph which shows the relationship between the offset voltage Voff and the image density on a paper surface recessed part on the conditions of return time ratio = 20 [%]. The graph which shows the relationship between the offset voltage Voff and the image density on a paper surface recessed part on the conditions of return time ratio = 50 [%]. The graph which shows the relationship between IDmax value and the frequency f of an alternating current component. The wave form diagram which shows the voltage waveform of the secondary transfer bias which has caused overshoot and undershoot. The wave form diagram which shows the voltage waveform of the secondary transfer bias which can improve an overshoot and an undershoot.

Hereinafter, an embodiment of an electrophotographic color printer (hereinafter simply referred to as a printer) will be described as an image forming apparatus to which the present invention is applied.
First, a basic configuration of the printer according to the embodiment will be described. FIG. 3 is a schematic configuration diagram illustrating the printer according to the embodiment. In the figure, the printer according to the embodiment includes four image forming units 1Y, 1M, 1C, and 1K for forming yellow (Y), magenta (M), cyan (C), and black (K) toner images. A transfer unit 30 as a transfer device, an optical writing unit 80, a fixing device 90, a paper feed cassette 100, and a registration roller pair 101.

  The four image forming units 1Y, 1M, 1C, and 1K use Y, M, C, and K toners of different colors as image forming materials, but other than that, they have the same configuration and are replaced when the lifetime is reached. Is done. Taking an image forming unit 1K for forming a K toner image as an example, as shown in FIG. 4, this includes a drum-shaped photosensitive member 2K as a latent image carrier, a drum cleaning device 3K, and a charge eliminating device (not shown). And a charging device 6K, a developing device 8K, and the like. These devices are held by a common holding body and integrally attached to and detached from the printer main body, so that they can be exchanged at the same time.

  The photoreceptor 2K has an organic photosensitive layer formed on the surface of a drum-shaped substrate, and is driven to rotate in the clockwise direction in the drawing by a driving means (not shown). The charging device 6K generates a discharge between the charging roller 7K and the photosensitive member 2K while bringing the charging roller 7K to which a charging bias is applied into contact with or in proximity to the photosensitive member 2K, thereby making the surface of the photosensitive member 2K uniform. Charge like this. In this printer, the toner is uniformly charged to the same negative polarity as the normal charging polarity of the toner. More specifically, it is uniformly charged to about −650 [V]. As the charging bias, one in which an AC voltage is superimposed on a DC voltage is employed. The charging roller 7K is formed by coating a metal cored bar with a conductive elastic layer made of a conductive elastic material. Instead of a method in which a charging member such as a charging roller is brought into contact with or close to the photoreceptor 2K, a method using a charging charger may be adopted.

  The uniformly charged surface of the photosensitive member 2K is optically scanned by a laser beam emitted from an optical writing unit, which will be described later, and carries an electrostatic latent image for K. The potential of the electrostatic latent image for K is about −100 [V]. The electrostatic latent image for K is developed by a developing device 8K using K toner (not shown) to become a K toner image. Then, primary transfer is performed on an intermediate transfer belt 31 described later.

  The drum cleaning device 3K removes transfer residual toner adhering to the surface of the photoreceptor 2K after the primary transfer step (primary transfer nip described later). It includes a cleaning brush roller 4K that is driven to rotate, a cleaning blade 5K that abuts the free end of the cleaning brush roller 4K in a cantilevered state, and the like. The transfer residual toner is scraped off from the surface of the photoreceptor 2K by the rotating cleaning brush roller 4K, and the transfer residual toner is scraped off from the surface of the photoreceptor 2K by the cleaning blade. The cleaning blade is in contact with the photosensitive member 2K in the counter direction in which the cantilevered support end side is directed downstream of the free end side in the drum rotation direction.

  The static eliminator neutralizes the residual charge on the photoreceptor 2K after being cleaned by the drum cleaning device 3K. By this charge removal, the surface of the photoreceptor 2K is initialized and prepared for the next image formation.

  The developing device 8K includes a developing unit 12K that includes the developing roll 9K, and a developer transport unit 13K that stirs and transports a K developer (not shown). The developer transport unit 13K includes a first transport chamber that houses the first screw member 10K and a second transport chamber that houses the second screw member 11K. Each of the screw members includes a rotary shaft member whose both ends in the axial direction are rotatably supported by bearings, and a spiral blade projecting in a spiral manner on the peripheral surface thereof.

  The first transfer chamber containing the first screw member 10K and the second transfer chamber containing the second screw member 11K are partitioned by a partition wall, but both ends of the partition wall in the screw axial direction. A communication port for communicating the two transfer chambers is formed at each location. The first screw member 10K is directed from the back side to the front side in the direction orthogonal to the paper surface in the drawing while stirring the K developer (not shown) held in the spiral blade in the rotation direction along with the rotation drive. Transport. Since the first screw member 10K and a later-described developing roll 9K are arranged in parallel so as to face each other, the transport direction of the K developer at this time is also a direction along the rotation axis direction of the developing roll 9K. . Then, the first screw member 10K supplies the K developer along the axial direction to the surface of the developing roll 9K.

  After the K developer transported to the vicinity of the front side end of the first screw member 10K enters the second transport chamber through the communication opening provided near the front end of the partition wall in the figure. The second screw member 11K is held in the spiral blade. Then, as the second screw member 11K is driven to rotate, the second screw member 11K is conveyed from the front side to the back side while being stirred in the rotation direction.

  In the second transfer chamber, a toner concentration sensor (not shown) is provided on the lower wall of the casing, and detects the K toner concentration of the K developer in the second transfer chamber. As the K toner density sensor, a sensor composed of a magnetic permeability sensor is used. Since the magnetic permeability of the K developer containing K toner and magnetic carrier has a correlation with the K toner concentration, the magnetic permeability sensor detects the K toner concentration.

  In this printer, Y, M, C, and K toner replenishing means (not shown) for individually replenishing Y, M, C, and K toners in the second storage chamber of the developing device for Y, M, C, and K, respectively. Is provided. The printer control unit stores Vtref for Y, M, C, and K, which are target values of output voltage values from the Y, M, C, and K toner density detection sensors, in the RAM. When the difference between the output voltage value from the Y, M, C, K toner density detection sensor and the Vtref for Y, M, C, K exceeds a predetermined value, the Y, M, C, K toner is detected for the time corresponding to the difference. M, C, K toner supply means is driven. As a result, Y, M, C, and K toners are replenished into the second transfer chamber of the developing device for Y, M, C, and K.

  The developing roll 9K accommodated in the developing unit 12K faces the first screw member 10K and also faces the photoreceptor 2K through an opening provided in the casing. The developing roll 9K includes a cylindrical developing sleeve made of a nonmagnetic pipe that is rotationally driven, and a magnet roller that is fixed inside the developing roll 9K so as not to rotate with the sleeve. Then, the K developer supplied from the first screw member 10K is carried on the sleeve surface by the magnetic force generated by the magnet roller, and is conveyed to the developing region facing the photoreceptor 2K as the sleeve rotates.

  A developing bias having the same polarity as the toner and larger than the electrostatic latent image of the photosensitive member 2K and smaller than the uniform charging potential of the photosensitive member 2K is applied to the developing sleeve. As a result, a developing potential for electrostatically moving the K toner on the developing sleeve toward the electrostatic latent image acts between the developing sleeve and the electrostatic latent image on the photoreceptor 2K. Further, a non-developing potential that moves K toner on the developing sleeve toward the sleeve surface acts between the developing sleeve and the background portion of the photoreceptor 2K. By the action of the developing potential and the non-developing potential, the K toner on the developing sleeve is selectively transferred to the electrostatic latent image on the photoreceptor 2K, and the electrostatic latent image is developed into the K toner image.

  3, the Y, M, and C image forming units 1Y, M, and C also have Y, M, and Y on the photoreceptors 2Y, M, and C in the same manner as the K image forming unit 1K. , C toner images are formed.

  Above the image forming units 1Y, 1M, 1C, and 1K, an optical writing unit 80 serving as a latent image writing unit is disposed. The optical writing unit 80 optically scans the photoreceptors 2Y, 2M, 2C, and 2K with laser light emitted from a laser diode based on image information sent from an external device such as a personal computer. By this optical scanning, electrostatic latent images for Y, M, C, and K are formed on the photoreceptors 2Y, M, C, and K. Specifically, the portion of the uniformly charged surface of the photoreceptor 2Y that has been irradiated with laser light attenuates the potential. Thereby, an electrostatic latent image is obtained in which the potential of the laser irradiation portion is smaller than the potential of the other portion (background portion). The optical writing unit 80 irradiates the photosensitive member through a plurality of optical lenses and mirrors while polarizing the laser light L emitted from the light source in the main scanning direction by a polygon mirror rotated by a polygon motor (not shown). To do. You may employ | adopt what performs optical writing by the LED light emitted from several LED of the LED array.

  Below the image forming units 1Y, 1M, 1C, and 1K, a transfer unit 30 is disposed as a transfer device that moves the endless intermediate transfer belt 31 endlessly in the counterclockwise direction in the drawing. . In addition to the intermediate transfer belt 31 as an image carrier, the transfer unit 31 includes a drive roller 32, a secondary transfer back roller 33, a cleaning backup roller 34, four primary transfer rollers 35Y, 35M, 35C, 35K, and a nip forming roller. 36, a belt cleaning device 37, and the like.

  The intermediate transfer belt 31 is stretched by a drive roller 32, a secondary transfer back roller 33, a cleaning backup roller 34, and four primary transfer rollers 35Y, 35M, 35C, and 35K disposed inside the loop. . Then, it is moved endlessly in the same direction by the rotational force of the driving roller 32 that is driven to rotate counterclockwise in the figure by a driving means (not shown).

  The four primary transfer rollers 35Y, 35M, 35C, and 35K sandwich an intermediate transfer belt 31 that can be moved endlessly between the photoreceptors 2Y, 2M, 2C, and 2K. As a result, primary transfer nips for Y, M, C, and K in which the front surface of the intermediate transfer belt 31 and the photoreceptors 2Y, 2M, 2C, and 2K abut are formed. A primary transfer bias is applied to the primary transfer rollers 35Y, 35M, 35C, and 35K by a transfer bias power source (not shown). As a result, a transfer electric field is formed between the Y, M, C, and K toner images on the photoreceptors 2Y, M, C, and K and the primary transfer rollers 35Y, 35M, 35C, and 35K. The Y toner formed on the surface of the Y photoconductor 2Y enters the Y primary transfer nip as the photoconductor 2Y rotates. Then, the image is primarily transferred from the photoreceptor 2Y to the intermediate transfer belt 31 by the action of the transfer electric field and nip pressure. The intermediate transfer belt 31 on which the Y toner image has been primarily transferred in this manner sequentially passes through the M, C, and K primary transfer nips. Then, the M, C, and K toner images on the photoreceptors 2M, C, and K are sequentially superimposed and superimposed on the Y toner image. A four-color superimposed toner image is formed on the intermediate transfer belt 31 by this superimposing primary transfer.

  The primary transfer rollers 35Y, 35M, 35C, 35K are made of an elastic roller having a metal cored bar and a conductive sponge layer fixed on the surface thereof. The axial centers of the primary transfer rollers 35Y, 35M, 35C, 35K are shifted to the downstream side in the belt moving direction by about 2.5 [mm] with respect to the shafts of the photoreceptors 2Y, M, C, K. In addition, primary transfer rollers 35Y, 35M, 35C, and 35K are disposed. A primary transfer bias is applied to such primary transfer rollers 35Y, 35M, 35C, 35K by constant current control. In place of the primary transfer rollers 35Y, 35M, 35C, 35K, a transfer charger or a transfer brush may be employed.

  The nip forming roller 36 of the transfer unit 30 is disposed outside the loop of the intermediate transfer belt 31, and the intermediate transfer belt 31 is sandwiched between the secondary transfer back roller 33 inside the loop. As a result, a secondary transfer nip where the front surface of the intermediate transfer belt 31 and the nip forming roller 36 abut is formed. While the nip forming roller 36 is grounded, a secondary transfer bias is applied to the secondary transfer back roller 33 by a secondary transfer bias power source 39. As a result, a secondary transfer electric field is formed between the secondary transfer back roller 33 and the nip forming roller 36 to electrostatically move the negative polarity toner from the secondary transfer back roller 33 side toward the nip forming roller 36 side. Is done.

  Below the transfer unit 31, a paper feed cassette 100 that stores a plurality of recording papers P in a stacked state is disposed. In this paper feed cassette 100, a paper feed roller 100a is brought into contact with the top recording paper P of the paper bundle, and this recording paper P is fed into the paper feed path by being driven to rotate at a predetermined timing. Send it out. A registration roller pair 101 is disposed near the end of the paper feed path. The registration roller pair 101 stops the rotation of both rollers as soon as the recording paper P delivered from the paper feed cassette 100 is sandwiched between the rollers. Then, rotation driving is resumed at a timing at which the sandwiched recording paper P can be synchronized with the four-color superimposed toner image on the intermediate transfer belt 31 in the secondary transfer nip, and the recording paper P is directed to the secondary transfer nip. Send it out. The four-color superimposed toner image on the intermediate transfer belt 31 brought into intimate contact with the recording paper P at the secondary transfer nip is secondarily transferred onto the recording paper P by the action of the secondary transfer electric field and nip pressure. Combined with the white color of P, a full color toner image is obtained. The recording paper P having the full-color toner image formed on the surface in this manner is separated from the nip forming roller 36 and the intermediate transfer belt 31 by the curvature when passing through the secondary transfer nip.

  The secondary transfer back roller 33 includes a cored bar and a conductive NBR rubber layer coated on the surface thereof. The nip forming roller 36 also includes a cored bar and a conductive NBR rubber layer coated on the surface thereof.

  The secondary transfer bias power supply 39 as a transfer bias output means has a DC power supply and an AC power supply, and can output a secondary transfer bias in which an AC voltage is superimposed on a DC voltage. Instead of grounding the nip forming roller 36 while applying the superimposed bias to the secondary transfer back surface roller 33, the secondary transfer back surface roller 33 may be grounded while applying the superimposed bias to the nip forming roller 36. . In this case, the polarity of the DC voltage is varied. Specifically, as shown in the figure, when a superimposed bias is applied to the secondary transfer back surface roller 33 under the condition that negative polarity toner is used and the nip forming roller 36 is grounded, the DC voltage is the same as that of the toner. Using the polarity, the time average potential of the superimposed bias is set to the same negative polarity as that of the toner. On the other hand, when the secondary transfer back surface roller 33 is grounded and the superimposed bias is applied to the nip forming roller 36, a DC voltage having a positive polarity opposite to that of the toner is used, and the time average of the superimposed bias is used. Is set to a positive polarity opposite to that of the toner. Instead of applying the superimposed bias to the secondary transfer back roller 33 or the nip forming roller 36, a DC voltage may be applied to one of the rollers and an AC voltage may be applied to the other roller. In addition, when using a recording paper P having a small surface unevenness such as plain paper without using a large surface unevenness such as rough paper, a shading pattern that follows the uneven pattern does not appear. A transfer bias composed only of a DC voltage may be applied. However, when using a paper with large surface irregularities such as rough paper, it is necessary to switch the transfer bias from a DC voltage only to a superimposed bias.

  Untransferred toner that has not been transferred to the recording paper P adheres to the intermediate transfer belt 31 after passing through the secondary transfer nip. This is cleaned from the belt surface by a belt cleaning device 37 in contact with the front surface of the intermediate transfer belt 31. A cleaning backup roller 34 disposed inside the loop of the intermediate transfer belt 31 backs up the cleaning of the belt by the belt cleaning device 37 from the inside of the loop.

  A fixing device 90 is disposed on the right side of the secondary transfer nip in the drawing. The fixing device 90 forms a fixing nip with a fixing roller 91 containing a heat source such as a halogen lamp and a pressure roller 92 that rotates while contacting with the fixing roller 91 with a predetermined pressure. The recording paper P fed into the fixing device 90 is sandwiched between the fixing nips in such a posture that the unfixed toner image carrying surface is in close contact with the fixing roller 91. Then, the toner in the toner image is softened by the influence of heating and pressurization, and the full color image is fixed. The recording paper P discharged from the fixing device 90 passes through a post-fixing conveyance path and is then discharged outside the apparatus.

  In this printer, the process linear velocity (the linear velocity of the photosensitive member and the intermediate transfer belt) in the standard mode is about 280 [mm / s]. However, the process linear velocity in the high image quality mode that prioritizes higher image quality than the print speed is set to a value slower than the standard mode. Further, the process linear velocity in the high speed mode in which the print speed is prioritized over the image quality is set to a value faster than that in the standard mode. Switching between the standard mode, the high image quality mode, and the high speed mode is performed by a key operation on a user operation panel or a printer property menu in a personal computer.

  In the case of forming a monochrome image, a support plate (not shown) supporting the primary transfer rollers 35Y, 35M, 35C for Y, M, and C in the transfer unit 30 is moved to move the primary transfer rollers 35Y, 35M. , C, K are moved away from the photoreceptors 2Y, M, C. As a result, the front surface of the intermediate transfer belt 31 is separated from the photoconductors 2Y, 2M, and 2C, and the intermediate transfer belt 31 is brought into contact with only the K photoconductor 2K. In this state, of the four image forming units 1Y, 1M, 1C, and 1K, only the K image forming unit 1K is driven to form a K toner image on the photoreceptor 2K.

  The secondary transfer bias power supply 39 outputs a secondary transfer bias composed of a DC component and an AC component. In this printer, the DC component of the secondary transfer bias has the same value as the offset voltage Voff that is the voltage of the DC component. In this printer in which the secondary transfer bias is applied to the secondary transfer back roller 33 and the nip forming roller 36 is grounded, the secondary transfer bias is in the secondary transfer nip when the polarity of the secondary transfer bias is the same negative polarity as the toner. , The negative polarity toner is electrostatically pushed out from the secondary transfer back surface roller 33 side to the nip forming roller 36 side. As a result, the toner on the intermediate transfer belt 31 is transferred onto the recording paper P. On the other hand, when the polarity of the superimposed bias is a positive polarity opposite to that of the toner, the negative polarity toner is electrostatically transferred from the nip forming roller 36 side to the secondary transfer back roller 33 side in the secondary transfer nip. To draw. As a result, the toner transferred to the recording paper P is attracted again to the intermediate transfer belt 31 side.

Next, observation experiments conducted by the present inventors will be described.
The present inventors manufactured a special observation experimental apparatus in order to observe the behavior of the toner in the secondary transfer nip. FIG. 5 is a schematic configuration diagram showing the observation experimental apparatus. This observation experimental apparatus includes a transparent substrate 210, a developing device 231, a Z stage 220, an illumination 241, a microscope 242, a high-speed camera 243, a personal computer 244, and the like. The transparent substrate 210 includes a glass plate 211, a transparent electrode 212 made of ITO (Indium Tin Oxide) formed on the lower surface thereof, and a transparent insulating layer 213 made of a transparent material coated on the transparent electrode 212. doing. The transparent substrate 210 is supported at a predetermined height by substrate support means (not shown). This substrate support means can be moved in the vertical and horizontal directions in the drawing by a moving mechanism (not shown). In the illustrated example, the transparent substrate 210 is positioned on the Z stage 220 on which the metal plate 215 is placed. However, the movement of the substrate support means causes the developing device 231 disposed on the side of the Z stage 220 to move. It is also possible to move directly above. The transparent electrode 212 of the transparent substrate 212 is connected to an electrode fixed to the substrate support means, and this electrode is grounded.

  The developing device 231 has the same configuration as the developing device of the printer according to the embodiment, and includes a screw member 232, a developing roll 233, a doctor blade 234, and the like. The developing roll 233 is rotationally driven in a state where a developing bias is applied by the power source 235.

  When the transparent substrate 210 is moved at a predetermined speed to a position directly above the developing device 231 and facing the developing roll 233 via a predetermined gap by the movement of the substrate support means, the toner on the developing roll 233 Is transferred onto the transparent electrode 212 of the transparent substrate 210. As a result, a toner layer 216 having a predetermined thickness is formed on the transparent electrode 212 of the transparent substrate 210. The toner adhesion amount per unit area with respect to the toner layer 216 includes the developer toner density, the toner charge amount, the developing bias value, the gap between the substrate 210 and the developing roll 233, the moving speed of the transparent substrate 210, and the rotation of the developing roll 233. The speed can be adjusted.

  The transparent substrate 210 on which the toner layer 216 is formed is translated to a position facing the recording paper 214 attached to the planar metal plate 215 with a conductive adhesive. The metal plate 215 is installed on a substrate 221 provided with a weight sensor, and the substrate 221 is installed on the Z stage 220. The metal plate 215 is connected to the voltage amplifier 217. A transfer bias composed of a DC voltage and an alternating voltage is input to the voltage amplifier 217 by the waveform generator 218, and a transfer bias amplified by the voltage amplifier 217 is applied to the metal plate 215. When the Z plate 220 is driven and controlled to raise the metal plate 215, the recording paper 214 starts to contact the toner layer 216. When the metal plate 215 is further raised, the pressure on the toner layer 216 increases, but the rise of the metal plate 215 is stopped so that the output from the weight sensor becomes a predetermined value. With the pressure set to a predetermined value, a transfer bias is applied to the metal plate 215 to observe the behavior of the toner. After the observation, the Z stage 220 is driven and controlled, the metal plate 215 is lowered, and the recording paper 214 is separated from the transparent substrate 210. Then, the toner layer 216 is transferred onto the recording paper 214.

  The behavior of the toner is observed using a microscope 242 and a high-speed camera 243 disposed above the substrate 210. Since the substrate 210 includes all of the glass plate 211, the transparent electrode 212, and the transparent insulating layer 213 made of a transparent material, the toner on the lower side of the transparent substrate 210 through the transparent substrate 210 from above the transparent electrode 210. Can be observed.

  As the microscope 242, a zoom lens made of KEYENCE zoom lens VH-Z75 was used. As the high-speed camera 243, FASTCAM-MAX 120KC manufactured by Photoron Co. was used. Photolon FASTCAM-MAX 120KC is driven and controlled by a personal computer 244. The microscope 242 and the high-speed camera 243 are supported by camera support means (not shown). This camera support means is configured so that the focus of the microscope 242 can be adjusted.

  The behavior of the toner on the transparent substrate 210 was photographed as follows. That is, first, the illumination light irradiates the observation position of the toner behavior with the illumination 241 to adjust the focus of the microscope 242. Next, a transfer bias is applied to the metal plate 215 to move the toner of the toner layer 216 attached to the lower surface of the transparent substrate 210 toward the recording paper 214. The behavior of the toner at this time was photographed with a high-speed camera 243.

Since the observation experimental apparatus shown in FIG. 5 and the printer according to the embodiment have different transfer nip structures for transferring toner to recording paper, the transfer electric field acting on the toner is different even if the transfer bias is the same. . In order to investigate the appropriate observation conditions, we also examined the transfer bias conditions that give good recess density reproducibility even with an observation experimental apparatus. As the recording paper 214, what is called FC Japanese paper type “Sazanami” manufactured by NBS Ricoh Co., Ltd. was used. As the toner, a Y toner having an average particle diameter of 6.8 [μm] mixed with a small amount of K toner was used. Since the observation experiment apparatus is configured to apply a transfer bias to the back surface of the recording paper (ripple wave), the polarity of the transfer bias capable of transferring the toner to the recording paper is opposite to that of the printer according to the embodiment. (Ie, positive polarity). As the alternating current component of the transfer bias composed of the superimposed bias, one having a sinusoidal waveform was adopted. The frequency f of the AC component is set to 1000 [Hz], the DC component (corresponding to the offset voltage Voff in this example) is set to 200 [V], and the peak-to-peak voltage Vpp is set to 1000 [V]. The toner layer 216 was transferred with a toner adhesion amount of 4 to 0.5 [mg / cm ² ]. As a result, a sufficient image density could be obtained on the concave portion on the surface of the “ripple”.

  At that time, the microscope 242 was focused on the toner layer 216 on the transparent substrate 210, and the behavior of the toner was photographed. Then, the following phenomenon was observed. That is, the toner particles in the toner layer 216 reciprocate between the transparent substrate 210 and the recording paper 214 due to an alternating electric field formed by the alternating current component of the transfer bias. The amount of toner particles to increase. Specifically, in the transfer nip, every time one period (1 / f) of the AC component of the transfer bias arrives, the alternating electric field acts once and the toner particles reciprocate once. In the first cycle, as shown in FIG. 6, only the toner particles present on the surface of the toner layer 216 are detached from the layer. Then, after entering the concave portion of the recording paper 216, it returns to the toner layer 216 again. At this time, the returned toner particles collide with the toner particles of the toner layer 216, thereby weakening the adhesion between the latter toner particles and the toner layer 216 or the transparent substrate 210. As a result, in the next cycle, as shown in FIG. 7, more toner particles are detached from the toner layer 216 than in the previous cycle. Then, after entering the concave portion of the recording paper 216, it returns to the toner layer 216 again. At this time, the returned toner particles collide with the toner particles still remaining in the toner layer 216, thereby weakening the adhesion between the latter toner particles and the toner layer 216 or the transparent substrate 210. As a result, in the next one cycle, as shown in FIG. 8, more toner particles are detached from the toner layer 216 than in the previous one cycle. In this way, the number of toner particles gradually increases each time they reciprocate. Then, it was found that when the nip passage time has elapsed (when the time corresponding to the nip passage time has elapsed in the observation experimental apparatus), a sufficient amount of toner has been transferred into the concave portion of the recording paper P.

  Next, when the DC behavior (corresponding to the offset voltage Voff in this example) is set to 200 [V] and the peak-to-peak voltage Vpp is set to 800 [V], the behavior of the toner is photographed. The following phenomenon was observed. That is, among the toner particles in the toner layer 216, those existing on the surface of the layer separate from the layer in the first one cycle and enter the concave portion of the recording paper P. However, the toner particles that entered entered the recesses without going to the toner layer 216. When the next period arrived, very few toner particles were newly detached from the toner layer 216 and entered into the recesses of the recording paper P. Therefore, when the nip passage time has elapsed, only a small amount of toner particles have been transferred into the recesses of the recording paper P. As a result of further experiments, the inventors of the present invention have found that the toner particles that have entered the concave portion of the recording paper P from the toner layer 216 in the first cycle can be returned to the toner layer 216 again. It was found that the value of Vr depends on the toner adhesion amount per unit area on the transparent substrate 210. As the toner adhesion amount on the transparent substrate 210 increases, the return peak value Vr at which the toner particles in the recesses of the recording paper P can be pulled back to the toner layer 216 increases.

Next, a characteristic configuration of the printer will be described.
FIG. 9 is a block diagram showing a part of the electric circuit of the printer. In the figure, a control unit 60 constituting a part of the transfer bias output means includes a CPU 60a (Central Processing Unit) as a calculation means, a RAM 60c (Random Access Memory) as a nonvolatile memory, and a ROM 60b (Read Only Memory) as a temporary storage means. And a flash memory 60d. Although various devices and sensors are connected to the control unit 60 that controls the entire apparatus, only devices and sensors related to the characteristic configuration of the printer are shown.

  The primary transfer power supplies 81Y, 81M, 81C, 81K output a primary transfer bias to be applied to the primary transfer rollers 35Y, 35M, 35C, 35K. The secondary transfer power supply 39 outputs a secondary transfer bias to be applied to the secondary transfer back surface roller 33, and constitutes a transfer bias output means together with the control unit 60. The operation panel 50 includes a touch panel (not shown) and a plurality of key buttons. The operation panel 50 displays an image on the screen of the touch panel and accepts an input operation by an operator using the touch panel and key buttons. An image can be displayed on the touch panel based on a control signal sent from the control unit 60.

  FIG. 10 is a waveform diagram showing the voltage waveform of the secondary transfer bias output from the secondary transfer power supply 39 shown in FIG. In the figure, the offset voltage Voff [V] represents the time average value of the secondary transfer bias voltage. As shown in the figure, in the secondary transfer bias composed of a superposed bias, the AC component is composed of repetition of a rectangular rising pulse and a rectangular falling pulse. Of the rising pulse and falling pulse, the rising pulse generates an electric field in the secondary transfer nip in the direction in which the toner charged to the normal polarity (minus in this example) is moved from the nip forming roller 36 side to the belt side. Form. On the other hand, the falling pulse forms an electric field in the secondary transfer nip in the direction in which the negatively charged toner is moved from the belt side to the nip forming roller 36 side. In the illustrated AC component, the falling time t2 that is the duration of the falling pulse is longer than the rising time t1 that is the duration of the rising pulse.

  If a rectangular pulse as shown in the figure is used as the AC component of the secondary transfer bias, it reaches the positive peak at the moment when the voltage is raised to the positive side, and reaches the negative peak at the moment when the voltage is lowered to the negative side. Therefore, the time to reach the peak is made significantly longer than the sine wave. As a result, it is possible to reciprocate the toner between the belt and the concave portion of the recording paper surface with a lower peak-to-peak voltage Vpp as compared with the case of using a conventional sine wave.

  In this printer, the falling edge of the falling pulse, which is the one that moves the negatively charged toner of the normal polarity from the belt side to the recording paper side, out of the rising pulse and the falling pulse of the AC component The time t2 is set longer than the rise time t1. Thereby, compared with the case where both time is made the same mutually, the feed peak value Vt is made lower. Specifically, with regard to the feed peak value, even when a large amount of toner enters the secondary transfer nip, such as when outputting a solid image, the large amount of toner is transferred well from the belt side to the recording paper side. It is necessary to set the conditions that can be allowed. When the fall time t2 is made relatively short, it is necessary to make the condition that a large amount of toner can be transferred from the belt surface to the recording paper P in a short time. Therefore, the feed peak value Vt needs to be made relatively large. On the other hand, when the fall time t2 is made relatively long, a sufficient time is required for transferring a large amount of toner, so that the feed peak value Vt can be made relatively small. For this reason, by making the fall time t2 longer than the rise time t1, the feed peak value Vt can be made lower than when both times are made the same. The return peak value Vr needs to be higher than when both times are the same, but the amount may be smaller than the amount by which the feed peak value Vt is reduced. This is because it is not necessary for the toner transferred to the recording paper P to be entirely returned to the toner layer on the belt by the rising pulse, and it is sufficient if a certain amount of the toner collides with the toner layer. For this reason, the peak-to-peak voltage Vpp which is the sum of the feed peak value vt and the return peak value vr can be further reduced by making the fall time t2 longer than the rise time t1.

  As a result of the above, in this printer, the toner in the secondary transfer nip is reciprocated at a lower peak-to-peak voltage Vpp than in the conventional case, so that a sufficient image can be obtained on the concave and convex portions on the surface of the recording paper. The generation of white spots can be suppressed while obtaining the density.

Next, an experiment conducted by the present inventors and a further characteristic configuration of the printer according to the embodiment will be described.
[Experiment 1]
The inventors prepared a print tester having the same configuration as the printer according to the embodiment. Various print tests were performed using this print tester. The process linear velocity, which is the linear velocity of the photoreceptor and the intermediate transfer belt 31, was set to 173 [mm / s]. The frequency f of the AC component of the secondary transfer bias was set to 1000 [Hz]. Further, as the recording paper P, RESAK 66 (trade name) 175 kg paper (a sixty-six continuous quantity) manufactured by Tokushu Paper Co., Ltd. was used. The resac 66 is paper having a degree of unevenness on the paper surface larger than “ripple waves”. The depth of the concave portion on the paper surface is about 100 [μm] at the maximum. A blue solid image obtained by superimposing the M solid image and the C solid image was output to the Rezac 66 under various secondary transfer bias conditions. Then, the image density (ID) of the M component and the image density (ID) of the C component of the output blue solid image were measured by X-Rite 938 manufactured by X-Rite, respectively. The sum of these two image densities was obtained as the blue image density. Blue color has an image density (ID) of 2.7 or higher, and most observers recognize that the color is sufficient. Therefore, the target value of the blue image density (ID) is set to 2.7 or more. Then, it was found that the conditions for the secondary transfer bias that can achieve the target image density of 2.7 or higher are relatively limited.

  As the return time ratio of the AC component of the secondary transfer bias, two conditions of 20 [%] and 50 [%] were adopted, but with each return time ratio, an offset for obtaining a target image density in blue. It was found that the value of the voltage Voff is greatly different. The return time ratio is the ratio of the duration of the pulse having the return peak value Vr to the sum of the rise time t1 and the fall time t2. In this example, it is the ratio of the rise time t1 to the sum, and is obtained by the equation “t1 / (t1 + t2) × 100 [%]”.

[Experiment 2]
Under the two conditions of return time ratio = 20 [%] and return time ratio = 50 [%], the offset voltage Voff at which the highest image density (blue ID) was obtained in the previous test print, respectively. The offset voltage Voff was finely changed in the range of 1 [kV] around the center, and a blue solid image was output under each condition. Then, in the same manner as in the previous experiment 1, the blue image density (ID) was measured for each blue solid image. As the image density (ID), the image density on the convex part on the paper surface and the image density on the concave part were measured. The results are shown in FIG. 11, FIG. 12, FIG. 13, and FIG.

  As can be seen from these figures, the image density on the concave portion and the image density on the convex portion are both under the condition of the return time ratio = 20 [%] than the condition of the return time ratio = 50 [%]. It can be seen that the range of the offset potential Voff that realizes ID = 2.7 or more is widened. A similar experiment was conducted with the return time ratio = 40 [%]. Similarly, the range of the offset potential Voff that achieves an image density of 2.7 or higher is obtained under the condition of the return time ratio = 40 [%]. It became wide. However, when the return time ratio is set to 10 [%], the toner that has entered the recesses on the paper surface cannot be satisfactorily returned to the belt side, so that a sufficient image density cannot be obtained on the recesses.

  Therefore, in the printer according to the embodiment, as the transfer bias output unit, the AC component of the secondary transfer bias is output so that the return time ratio is higher than 10 [%] and lower than 50 [%]. The secondary transfer power supply 39 is configured.

[Experiment 3]
The inventors investigated the minimum value of the rise time t1 in which the toner that entered the concave portion on the paper surface can be effectively returned onto the belt in the secondary transfer nip. Specifically, the frequency f of the AC component of the secondary transfer bias, the offset voltage Voff, and the peak-to-peak voltage are appropriately changed under the conditions of the return time ratio = 20 [%] and 50 [%], respectively. The image density on the concave portion of the blue solid image under each condition was measured. FIG. 15 shows the relationship between the IDmax value obtained by this experiment and the frequency f of the AC component.

  As shown in the figure, basically, the higher the frequency f, the higher the IDmax value (maximum image density) of the image. This is because the frequency of reciprocating movement of the toner in the secondary transfer nip increases as the frequency f is increased. Under the condition of the return time ratio = 50 [%], the phenomenon that the IDmax value is increased as the frequency f is increased is recognized in the entire region of the band of 200 to 1500 [Hz]. The higher the frequency f, the shorter the rise time t1. Under the condition of the return time ratio = 50 [%] and the frequency f = 1500 [Hz], as shown in the figure, an IDmax value of 3 which is significantly larger than the target 2.7 is obtained. The rise time t1 under this condition is 0.33 [ms] (1 / 1.5 × 0.5), and this causes the toner in the concave portion of the paper surface to return to the belt side in the secondary transfer nip. It can be said that this is an effective value for the secondary reversion effective for.

  On the other hand, under the condition of the return time ratio = 20 [%], as shown in the figure, in the band of 200 to 1000 [Hz], the IDmax value is increased as the frequency f is increased, but the frequency f is increased from 1000 [Hz]. If it is increased, the IDmax value is decreased. The rise time t1 under the conditions of the return time ratio = 50 [%] and the frequency f = 1000 [Hz] is 0.2 [ms] (1/1 × 0.2). On the other hand, the rise time t1 under the condition of the return time ratio = 50 [%] and the frequency f = 1500 [Hz] is 0.13 [ms] (1 / 1.5 × 0.2). If the rise time t1 can only be secured to 0.13 [ms], the toner in the recesses on the paper surface cannot sufficiently return to the belt surface within that time, so the IDmax value seems to have decreased. Therefore, it is necessary to secure 0.2 [ms] or more for the duration of the pulse having the return peak value Vr (rising time t1 in this example).

  Therefore, in the printer according to the embodiment, the secondary transfer power supply 39 is configured to output an AC component having a rise time t1 of 0.2 [ms] or more.

[Experiment 4]
AC component frequency f and process linear velocity v under conditions of AC component peak-to-peak voltage Vpp = 2500 [V], offset voltage Voff = −800 [V], return time ratio = 20 [%] While changing, a blue solid image was output on plain paper under the conditions of f and v. The output solid image was visually observed. Then, the presence / absence of image density unevenness (pitch unevenness) considered to be an influence of an alternating electric field in the secondary transfer nip was evaluated. Then, under the condition of the same frequency f, it was found that as the process linear velocity v is increased, it becomes easier to generate pitch unevenness, and under the same process linear velocity v, it is easier to generate pitch unevenness as the frequency f is decreased. These results indicate that if the toner is not reciprocated between the belt and the concave portion of the paper surface by a certain number of times (hereinafter referred to as reciprocation number N in the nip) in the secondary transfer nip, pitch unevenness may occur. Is shown. No pitch unevenness was observed under the conditions of v = 282 [mm / s] and f = 400 [Hz], but pitch unevenness was observed under the conditions of V = 282 [mm / s] and f = 300 [Hz]. It was. The secondary transfer nip width d, which is the length of the secondary transfer nip in the belt moving direction, is 3 [mm]. Therefore, the number N of reciprocations in the nip under the above conditions is calculated to be about 4 times (3 × 400/282). With this value, pitch unevenness can be avoided at the last minute. Further, no pitch unevenness was observed under the conditions of v = 141 [mm / s] and f = 200 [Hz], but the pitch unevenness was satisfied under the conditions of V = 141 [mm / s] and f = 100 [Hz]. Was recognized. Similarly to the condition of v = 282 [mm / s] and f = 400 [Hz], the condition of v = 141 [mm / s] and f = 200 [Hz] is about 4 in the nip. Times (3 × 200/141. Therefore, it can be said that an image without pitch unevenness can be obtained by satisfying the condition “frequency f> (4 / d) * v”.

  Therefore, in the printer according to the embodiment, the secondary transfer power supply 39 is configured to output an AC component having a relationship of “f> (4 / d) × v”. In order to satisfy such conditions, the printer includes an operation panel 50 as information acquisition means and communication means for acquiring printer driver setting information sent from the outside through communication. Based on the obtained information, it is grasped whether the printing operation is performed in the high speed mode, the standard mode, or the low speed mode. Then, the process linear velocity v is grasped based on the grasp result.

[Experiment 5]
In the secondary transfer nip, the toner cannot be satisfactorily transferred unless a certain amount of transfer current flows to the recording paper P. Of course, a transfer current is less likely to flow for thick paper than for paper of general thickness. It is desirable that the toner adheres well to the protrusions and recesses on the paper surface, both for normal thickness paper and thick Japanese paper. Experiment 5 was conducted to examine how the secondary transfer bias is controlled to be advantageous.

As the secondary transfer power source 39, a power source that outputs the peak-to-peak Vpp of the AC component and the offset voltage Voff by constant voltage control was used. Various other conditions are as follows.
Process linear velocity v = 282 [mm / s].
Recording paper: 175 kg paper of Rezac 66
Test image: A4 size black solid image.
-Return time ratio = 40 [%].
・ Offset voltage Voff: 800 to 1800 [V]
-Peak-to-peak voltage Vpp: 3-8 [kV]
・ Frequency f = 500 [Hz]

The image density on the concave portion of the paper surface of the black solid image output under the above conditions was evaluated as follows.
Rank 5: The inside of the recess is completely filled with toner.
Rank 4: The concave portion is almost completely filled with toner, but the paper background is slightly visible at the portion where the depth of the concave portion is large. Rank 3: The paper background is clearly visible at the portion where the depth of the concave portion is large.
Rank 2: worse than rank 3 and better than rank 1 described later.
Rank 1: No toner adheres to the recess.

Further, the image density on the convex portion of the paper surface of the black solid image was evaluated as follows.
Rank 5: There is no density unevenness and a good image density is obtained.
Rank 4: Although there is a slight density unevenness, an image density with no problem even in a thin part is obtained.
Rank 3: There is density unevenness, and the image density in a thin area is insufficient beyond an allowable level.
-It is worse than rank 2 and better than rank 1 described later.
Rank 1: Overall image density is insufficient.

Then, the evaluation result of the image density on the concave portion and the evaluation result of the image density on the convex portion were integrated as follows.
・ ●: The evaluation results of the image density of the concave and convex portions are both rank 5 or higher.
○: The evaluation results of the image density of the concave and convex portions are both rank 4 or higher.
-: Only the evaluation result of the image density of the concave is rank 3 or lower.
Δ: Only the evaluation result of the image density of the convex portion is rank 3 or less.
*: The evaluation results of the image density of the concave and convex portions are both rank 3 or less.

  Next, a similar experiment was performed by replacing the recording paper P with a 215 kg paper with a thicker lazac 66 instead of the 175 kg paper with a lazac 66. For the combination of the offset voltage Voff and the peak-to-peak voltage Vpp, among all the combinations applied to the experiment, both ● (concave, convex) for both the resack 66 (175 kg paper) and the resack 66 (215 kg paper) Evaluation results of the image density of the part are both ranked 5 or higher), and results of ○ (the evaluation results of the image density of the concave and convex parts are both rank 4 or higher) are obtained. Extracted. As a result, there was no such combination that gave a result of ● on both papers. In addition, the combination in which a result of ◯ was obtained on both papers was Vpp = 6 [kV] and offset voltage Voff = −1100 ± 100 [V] (center value ± 9%).

[Experiment 6]
As the secondary transfer power source 39, one that outputs the offset voltage Voff by constant current control was used. The output target value (offset current Ioff) was set to -30 to -60 μA. The experiment was performed in the same manner as Experiment 5 except for the conditions other than these. As a result, the combination of Vpp and offset current Ipp that gives a result (●) that the evaluation results of the image density of the concave and convex portions are both rank 5 or higher is Vpp = 7 kV, Ioff = −42.5 ± 7. It was 5 [μA] (central value ± 18%). In addition, the combination in which a result of ◯ is obtained on both papers is Vpp = 7 kV, offset current Ioff = −47.5 ± 12.5 [μA] (center value ± 26%).

  Thus, in Experiment 5, there was no combination that resulted in ● for both papers, whereas in Experiment 6, there was a combination that resulted in ● for both papers. In addition, when focusing on the combination in which the result of ◯ is obtained, in Experiment 5, the offset voltage Voff = −1100 ± 100 [V] (center value ± 9%), whereas in Experiment 6, Vpp = 7 kV, Offset current Ioff = −47.5 ± 12.5 [μA] (center value ± 26%), and the latter clearly has a wider numerical range from the center value. These experimental results show that the constant current control increases the margin for setting the control target value that can be handled from paper with a normal thickness to thick paper, compared with the case where the DC component is controlled with constant voltage. That means getting.

  Therefore, in the printer according to the embodiment, a secondary transfer power supply 39 that outputs a direct current component by constant current control is used. Note that the secondary transfer power supply 39 outputs a peak-to-peak current with constant current control even for an AC component. Thereby, regardless of environmental fluctuations, by making the peak-to-peak current constant, an effective return peak current and feed peak current can be reliably generated.

  When an overshoot or undershoot as shown in FIG. 16 is generated in the voltage waveform of the secondary transfer bias, the return peak value Vr or the feed peak value Vt (current return in the case of constant current control) is only instantaneously generated. The peak value Ir and the feed peak value It) are made larger than the original. This may cause a slight white spot. Therefore, it is desirable to configure the secondary transfer power supply 39 so as to output a waveform having a rectangular corner chamfered as shown in FIG. By doing so, even if an overshoot or undershoot is caused, the return peak value or the feed peak value can be kept lower than the desired value. In the present invention, the rectangular wave means a pulse whose peak value is 60 [%] or more of the whole in the rising pulse and the falling pulse.

  As described above, in the printer according to the embodiment, as the AC component, the return time ratio, which is the “ratio of the duration of the other pulse” with respect to the sum of the rise time t1 and the fall time t2, is higher than 10%. The secondary transfer power supply 39, which is a transfer bias output means, is configured to output a voltage lower than 50 [%]. In such a configuration, the following merit can be obtained as compared with the case where the return time ratio is 50% or more. That is, a set value for setting a value that realizes a sufficient image density as an output voltage target value when the offset voltage Voff is controlled at a constant voltage or an output current target value when the offset current Ioff is controlled at a constant current. The margin can be increased. In addition, by setting the return time ratio larger than 10 [%], a sufficient image density can be obtained on the concave portion of the paper surface.

  In the printer according to the embodiment, the secondary transfer power supply 39 is configured to output an alternating current component having a rise time t1 of 0.2 [ms] or more. In this configuration, as described above, it is possible to avoid the occurrence of insufficient image density on the concave portion of the paper surface due to the rise time t1 being too short.

  In the printer according to the embodiment, “f> (4 / d) regarding the frequency f [Hz], the secondary transfer nip width d [mm], and the process linear velocity v [mm / s] as AC components. The secondary transfer power supply 39 is configured so as to output a signal having a relationship of “) × v”. In such a configuration, as already described, the toner can be reciprocated four or more times in the secondary transfer nip, thereby avoiding occurrence of pitch unevenness.

  Further, in the printer according to the embodiment, the secondary transfer power supply 39 is configured so as to perform the process of outputting the direct current component by constant current control. In such a configuration, as described above, it is possible to increase a margin for setting a control target value that can be handled from a sheet having a normal thickness to a thick sheet as compared with the case where the DC component is controlled at a constant voltage.

  Further, in the printer according to the embodiment, the secondary transfer bias power supply 39 is configured so as to perform the process of performing constant current control on the output value of the peak-to-peak current of the AC component. In such a configuration, it is possible to reliably generate an effective return peak current and feed peak current by making the peak-to-peak current constant regardless of environmental fluctuations.

30: Transfer unit (transfer device)
31: Intermediate transfer belt (image carrier)
36: Nip forming roller (nip forming member)
39: Secondary transfer power supply (transfer bias output means)

JP 2006-267486 A

Claims (8)

A nip forming member that forms a transfer nip by contacting a front surface of the image carrier carrying a toner image; and a toner image on the image carrier against a recording material sandwiched in the transfer nip. In a transfer apparatus having a transfer bias output means for outputting a transfer bias including a direct current component and an alternating current component in order to transfer,
As the transfer bias ,
Ri repeated Do from the rectangular rising pulse and a rectangular shape of the falling pulse,
The bias polarity is switched multiple times while the recording material is passed through the transfer nip,
Among the previous SL rising pulse and the falling pulse, one pulse electric field direction is towards the formation in the nip to move the charged toner to the normal polarity from the image bearing member to the nip forming member side The duration of is longer than the duration of the other pulse that moves the toner image charged to the normal polarity from the nip forming member side to the image carrier side ,
The combination of the duration of the one pulse, the magnitude of the one pulse, the duration of the other pulse, and the magnitude of the other pulse is charged to a normal polarity in the transfer nip. A transfer apparatus, wherein the transfer bias output means is configured to output a toner that is a combination of relatively moving toner from the image carrier side to the nip forming member side . The transfer device according to claim 1.
As the AC component, the ratio of the duration of the other pulse to the sum of the duration of the rising pulse and the duration of the falling pulse is higher than 10 [%] and lower than 50 [%] The transfer apparatus is characterized in that the transfer bias output means is configured so as to output the output. The transfer device according to claim 2.
The transfer apparatus according to claim 1, wherein the transfer bias output means is configured to output the AC component having a duration of the other pulse of 0.2 [ms] or more. The transfer device according to any one of claims 1 to 3,
As the AC component, a frequency f [Hz], a nip width d [mm] which is the length of the transfer nip in the image carrier surface movement direction, and a surface movement speed v [mm / s] of the image carrier. A transfer apparatus, wherein the transfer bias output means is configured to output a signal having a relationship of “f> (4 / d) × v”. In the transfer device according to any one of claims 1 to 4,
The transfer apparatus, wherein the transfer bias output means is configured to perform a process of outputting the DC component by constant current control. The transfer device according to claim 5.
The transfer apparatus, wherein the transfer bias output means is configured to perform a constant current control on an output value of a peak-to-peak current of the AC component. An image carrier for carrying a toner image;
In an image forming apparatus comprising: a transfer device that transfers a toner image on the surface of the image carrier to a recording material;
An image forming apparatus using the transfer device according to claim 1 as the transfer device. The image forming apparatus according to claim 7.
An image forming apparatus, wherein the recording material has an uneven surface.

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